This relates generally to image sensors, and more specifically, backside illuminated image sensors with improved surface planarity.
Modern electronic devices such as cellular telephones, cameras, and computers often use digital image sensors. Imagers (i.e., image sensors) include a two-dimensional array of image sensing pixels. Each pixel includes a photosensor such as a photodiode that receives incident photons (light) and converts the photons into electrical charges. Conventional image pixel arrays include frontside illuminated image pixels or backside illuminated image pixels. Image pixels are fabricated on a semiconductor substrate using complementary metal-oxide-semiconductor (CMOS) technology or charge-coupled device (CCD) technology.
In conventional backside illuminated image pixels, bond pads are often formed above the surface of the substrate at the periphery of the pixel region, which increases surface topography. High surface topography can negatively impact device yield by causing window framing (i.e., nonlinearity at edges of the image sensor), streak defects and shading defects during subsequent processing of the color filter array (CFA) and microlenses. For example, in conventional fabrication of color filter array elements and microlenses on a wafer, resist spin coating operations can sometimes result in regions of the wafer having an overly thin or overly thick covering of photoresist (commonly referred to as streaking). This streaking results from high levels of deviation in the depth of surface features such as bond pads, trenches, and recessed arrays. The deviation in surface topography prevents resist from spreading uniformly across the surface of the wafer (e.g., features that protrude above or that create a depression in the wafer surface block the flow of resist). Such streaking results in an undesirable reduction in device quality and, consequently, reduced device yield.
Some conventional backside illuminated image sensors have bond pads that are partially recessed in a substrate, where a top surface of each bond pad extends above a top surface of the substrate. Such bond pads are connected to through-silicon via structures through a metal layer on the top surface of the substrate, where the through-silicon via structures are adjacent to the bond pads. These image sensor bond pads contribute to undesirable surface topography as described above because they extend above the surface of the substrate. Other conventional image sensors have bond pads that are recessed into a substrate such that a top surface of each bond pad is below a top surface of the substrate. These image sensor bond pads also contribute to undesirable surface topography as described above because the top surface of each bond pad does not extend to the top surface of the substrate and thereby creates a depression in the surface of the image sensor.
Alignment structures for use in photolithographic processing are sometimes formed in the scribe-line area between adjacent dies. In conventional backside illuminated image sensors, it is necessary to create openings in the scribe line substrate so that the alignment structures are visible from the backside. This process further contributes to high topography.
It would therefore be desirable to provide image sensors with improved surface planarity in areas in which bond pads and alignment structures are formed.